The resin from which the lake is made determines how it dries

I told you at one point that varnishes must be chosen according to their properties. For a table, for example, we are very interested in the varnish's resistance to aggression (scratches, stains, knocks), whereas for other pieces of furniture this is not so important. An outdoor wooden object will have to withstand the action of environmental factors, whereas an indoor wooden object has no such problems. These properties are given by the type of varnish, the resin from which it is made. The resin also determines how the varnish dries. The conditions for a natural drying Correctly described in another article, let's see how the varnish dries depending on the resin that determines its characteristics.

Depending on the nature of the resin, drying can be of 3 types: physical drying, chemical and combined.

Physical drying

Physical drying is specific to nitrocellulose products - one of the oldest, simplest and easiest to apply varnishes. The base resin, nitrocellulose, is dissolved in a mixture of solvents. After application, the varnish dries by simply evaporating these solvents. The process is reversible, i.e. if thinner is put on the varnish film it will dissolve and become liquid again.

Reversibility is a disadvantage in terms of film strength, but an advantage if you want to refinish for various reasons. The lacquer film does not resist if you spill a glass of alcohol on it, but if there are flaws or you want to replace the finish, the lacquer film will wash with thinner.

Chemical drying

When the film is the result of a chemical reaction between two components A and B we talk about chemical drying. This is the case for polyurethane, acrylic with hardener or polyester products. If you decide to work with such products, you should be aware that the reaction starts as soon as the hardener (component B) is mixed with the base resin (component A). Drying is not instantaneous, however, as it will take time to apply the mixture.

The time during which the mixture is liquid and of constant viscosity is called pot-life. It is not the same for all products and differs from product to product. It can be from 15-20 minutes to 6-8 hours or more. After this time, the mixture should not be applied even if it is still liquid, as it loses its properties. Its viscosity increases and the temptation to dilute it for further use should be resisted. Why? Because the reaction has started in the mixture in the meantime and the diluent cannot dissolve the compound formed, the reaction is irreversible.

The advantage of using these types of products is the very good film strength. The disadvantages start with pot-life. That is, if more than the required amount is mixed in, the remaining difference is lost if not used within the pot-life limit. Problems can also arise if the mixing of the components is not done correctly according to the directions specified in the data sheets or on the packaging.

If you forget to add hardener the film does not dry and remains tacky. If less catalyst is added the resulting film will be soft (no mechanical resistance) and if more is added the film will become brittle.

Combined drying

Drying of water soluble materials is an example of combined drying. Waterborne products are dispersions of acrylic, polyurethane or acrylic-polyurethane resins in water. As they are applied, the water evaporates and the molecules come together. This is the physical drying part. This is why I have always said that ventilation is very important with this type of product. It helps to remove the water as quickly as possible. When the molecules get close enough, they start to react with each other, forming the water-insoluble varnish film. This is the chemical part of drying.

Alkyd paints are also a combined drying. The film is formed by the evaporation of the solvent and the reaction between alkyd resin and oxygen in the air. As long as the paint is covered and does not come into contact with oxygen in the air, it remains liquid. When applied thinly, it removes the solvent more quickly and the large surface area in contact with the air speeds up drying. After use, containers must be tightly closed so that the paint remains liquid for as long as possible. But it will form a dry bead on top because air has already entered the container. This is why linseed oil is sometimes put on top of the paint to eliminate contact with the air in the can.

Only by choosing the right finishing materials can you be guaranteed to get a quality film. They must be applied and dried correctly, following the manufacturers' recommendations.

I hope you find the above information useful. As always, additions are welcome. And if you have any questions or queries, please leave them below in the space provided. I will be sure to reply.